Method of liquefying gases



1956 H. SIXSMITH 2,760,356

METHOD OF LIQUEFYING GASES Filed April 8, 1953 3 Sheets-Sheet l INVENTOQv HI mseaeer s/x SMITH 1956 H. SIXSMITH METHOD OF LIQUEFYING GASES 3Sheets-Sheet 2 Filed April 8, 1955 mm mm INVENTO? HEEBERT S/XSM/TH ATree vs YS Aug. 28, 1956 H. SIXSMITH 2,760,356

METHOD OF LIQUEFYING GASES Filed April 8, 1953 3 Sheets-Sheet 3 IN VEN70? #525527 SIXSM/Th AMMLK.

A TTOENEVS United States Patent 2,760,356 METHOD OF rroonuvnsn AsEsHerbert Sixsmith, Reading, England, assignor National ResearchDevelopment Gorp'oratiini, Boudoir, England, a corporafion of GreatBrit'ain Application April 8, 1953, Serial No. 347,513 Claim s priority,application GreatBi'itainAprilfZZ,1952 7' c ai or, 61-175 This inventionrelates to a method of and apparatus for' liquefying' gases, for exampleair. 1

It is'kriown to liquefy gases by compressing. the gas, expanding a partof it adiabatically, for exampl athi bin'e", and effecting heat exchangein a condenser between. the expanded and the unexpanded parts so as tocondense the latter, the whole of the compressed gas being first cooledby heat exchange with the expanded gas exhausted, from: the condenser.This method has the drawback that the. mass flow of warm gas through thefirst stage heatex changer or cooler is greater than that of the coldexpanded gas exhausted from the condenser, so that the temperaturedifference between the expanded part and the compressed part at'the coldend of the said cooler is undesirably large, resulting in a loss ofefiiciency.

Broadly stated, the invention consists ofarranging for the net mass flowof compressed gas through the first stage heat exchanger or cooler to besubstantially equalto the mass flow of expanded gas therethrough. Inthis way, maximum efiiciency of heat exchange is obtainable, and thetemperature difference between the incoming expanded gas and theefiluent compressed gas is Since, however, the flow of expanded gas isequal to the total available compressed gas less the fraction liquefied(neglecting any losses due to condensation of water vapour andsublimation of CO2), the above-mentioned balance of mass flow throughthe first stage cooler can; only be achieved if a proportion, equal tothe fraction to be liquefied, is divided off from the total availablecompressed gas flow and either caused to by-passthe cooleror, havingpassed through the cooler with the; remainder, is recirculatedtherethrough in the reverse sense. In the latter case, the recirculatingproportion isre-heated by the incoming gas, so that the net mass flow ofcompressed gas is substantially equal to the mass flow of expanded gas.

In the former case, the by-passing proportion may actually constitutethe fraction to be liquefied, being-supplied direct to the condenser;alternatively, the fraction tobe liquefied is split off after the firststage cooler, and the equivalent proportion which by-passes the cooleris re united with-the balance of the compressed gas after this splittingoil and before expansion. In the recirculat'ory system mentioned above,the fraction to be liquefied is also split off from the cooledcompressed gas, but after division therefrom of the recirculatoryproportion, and this pro portion now re-heated, is reunited with theresidual cold compressed eflluent from the cooler before expansion.

Where the gas to be liquefied isa permanent gas, for example air, it isdesirable to remove, at as early a stage as possible, thoseconstituents, such as water and/ or carbon dioxide, which solidify atthe low temperatures involved. It is therefore convenient to arrangethat the temperature at the cold end of the first stage cooler issuchthat the said constituents solidify in the cooler, and at leastthecompress'ed' efiluent from the cooler is Where, however, the fractionto be liquefied, or an ec uivalent proportion thereto, by-pa'sse's thecooler, it is preferable to 2,760,356 Eatent d u 1956.

ensure that the gas is dry either at the input tothe compressor or atthe point of division of the fraction to. be liquefied; or itsequivalent proportion. Thisprevents the deposition of ice: or solid COin the condenser on the one hand or in the turbine or other expansiondevice on the other. The recirculatory system mentioned above is thusseen to have an advantage over the other two alternatives according tothe present invention, in that no special; preliminary precautions arenecessary to dry the gas.

Where the gas issuing from the cooler is expandediin a turbine, thetemperature of this cooled gas is in, certain cases" lower'than that atwhich maximum thermal efiiciency can be obtained from the expansionstage. In such cases, it is preferred to adopt a system in which the;balancing of the mass flows is achieved by splitting oil? a proportionof the compressed gas equivalent to the fraction to be liquefied, thisequivalent proportion being already at, or then raised to, substantiallythe initial temperature and recombined with the cooled residual part ofthe gas which isfto be expahded' as this latter part passes tov theexpansion stage. The effect of this splitting oh and recombination' ofan equivalent proportion is to raise thetemperature of the. gas enteringthe expansion stage.

methods of the invention are efiiciently operable at eoin'par'ativelylowpressures and the compressed gas may, forexample, be at a pressure ofbetween 8 and 15, at rn'ospheres. l

' The heat exchanger used in the invention is preferably ofthe're'g'enerative type, in which a number of chambers or compartments;are successively opened to gas flow in appropriate directions bya' valvedevice moving relatively to the said chambers or'com'partinents(hereinafter called; chambers for convenience) or to ducts serving them.At least one chamber is provided for the or each stream oi compressedgas which is to fiow through the. regen-v erator; and for the stream ofexpanded gas. It is desirable that the total regenerator volumesprovided for the expanded and the compressed gas should be in inverser'atio' to the respective gas" pressures. This would, however, involveproviding an unduly large number of chambers, for the expanded gas. Aconvenient compromise where, for example, the compressed gas is at apressure of 8 atmospheres, is to provide two chambers for the expandedgas and one for the compressed gas being. cooled, Where the mode ofoperation is such that the by-p'assing part of the gas is passed throughthe regenorator in reverse direction to the main stream of con pressedgas, a furthercharnber is provided for this part, In addition, asdescribed below, it is convenient to provide an additional chamber,making five in all, in which, durin'g' any one regenerative step, therequisite pressure is built up to enable the reverse flow of by-passinggas to take place therein in the next succeeding regenerative step. 7

Where the adiabatic expansion is eficcted in a single stage turbine itis necessary that this shall rotate at extremely high speeds. In certaintypes of turbine the force of the air jets impinging on the rotor vaneshas a component capable of exciting radial oscillations of the rotorwhen adequate provision for the damping of such oscillations has notbeen made. The invention comprises a turbine adapted for use in theabove-described methods, in which at least one main bearing. for therotor shaft is mounted ouFa member which is freeto carry out-small ingcomponent of force of the air jets. It is furthermore an advantage ofthis arrangement that the amplitudes of oscillation of the rotor at thecritical speeds are greatly reduced. The rotor shaft may be made longand slender, thus having a comparatively low heat conductivity. Thebearings can thus be'maintained at a temperature sufficiently high toenable them to be lubricated. The turbine is preferably of the axialflow impulse type.

The invention as applied to the liquefaction of air'will now beparticularly described with reference to the ,accompanying drawings, inwhich:

Figure 1 is a diagram illustrating the method of the invention, and

Figures 2 and 3' are diagrams illustrating modifications; J

Figure 4 shows, in diagrammatic form, an apparatus for carrying out themethod of the invention as shown in Figure3; and

Figure 5 is a part sectional, part elevational view of a part of theaxial flow impulse turbine used in the apparatus of Figure 4.

In Figure 1, an input stream of air A from a compressor and pre-cooler(not shown) (at a pressure of, for example, 8 atmospheres) is dividedinto first and second fractions or streams a, b respectively, and thefirst fraction a is passed through a first heat exchange stage or coolerE1, from which it passes to an expansion turbine T provided with an airbrake T1. After leaving the expansion turbine, the expanded and cooledfraction enters a condenser E2 from which it flows in reverse directionthrough the first stage cooler E1 and thenceto atmosphere. Since theexpanded gas stream is wholly constituted by the compressed fraction 11(neglecting'losses due to condensation of water or sublimation of CO2)the etfective'mass flows in opposite directions through the first stagecooler E1 are substantially equal. The second fraction or stream benters the condenser'Ez at atmospheric temperature. The temperature ofthe first fraction a leaving the turbine may be approximately 80absolute, a temperature low enough to cause liquefaction of thecompressed second fraction b in the condenser E2. The liquid is expandedin a valve V1 to atmospheric pressure and collected in a reservoir Rfrom which it is tapped oif at intervals through a valve V2.

' In Figure 2, a subsidiary stream or proportion b, equivalent in massflow to the fraction 0 to be liquefied, is

a split off from the input stream A before the latter enters the coolerE1 and passes to the turbine T. After leaving the first stage 'coolerE1, the cooled compressed gas splits into the residual stream a and thefraction 0 respectively. The proportion b is then recombined with theresidual stream a. Thus, carbon dioxide and water vapour are removed inE1 from the residual stream a and the fraction c before passage throughthe turbine T and the con.- denser E2 respectively. Since the equivalentproportion b by-passes the first stage cooler E1, it serves to warm thefraction a so as to increase the expansion efliciency of the turbine.

In Figure 3, the equivalent proportion b is not split 01f from the inputstream A until this has passed through the first stage cooler E1. Carbondioxide and water vapour are thus removed from it, and it is passed inreverse direction back through E1 to be reheated before joining theresidual stream a entering the turbine T. The fraction 6 be liquefiedpasses to the receiver R through the condenser B2.

In each of Figures 1-3 the massflow of the stream b is substantiallyequal to that of the air withdrawn as a Connection to the 4 v in planalthough it is shown for convenience in a schematic quasi-developedform.

The main stream of air from a compressor 2, in which the air iscompressed to a pressure of, for example, 8 atmospheres, passes througha pre-cooling and cleaning stage 3 in which it is cooled to atmospherictemperature. In the position of the rotary valve 1 shown in Figure 4,the cooled compressed air passes by way of this valve, through the valveport P1, in to the regenerator chamber A1 in which it transfers its heatto the packing therein and is cooled nearly to the temperature of theexpanded air stream leaving the condenser in the manner to 'be laterdescribed.

The stream of cooled compressed air now passes via the valve Vs, themanifold 4, and a restriction R1 to the expansion turbine 5 in which itis cooled by adiabatic expansion. At D and B, respectively, anequivalent pro portion b and the fraction 0 to be liquefied leave themain stream and accordingly, between points B and C the streamconstitutes the residual stream a referred to in the description ofFigures 2 and 3. The cold exhaust air from the turbine 5 passes throughthe pipe 6 to a condenser 7, in which it absorbs heat from the fraction0 which reaches the condenser via pipe 8 and valve 9. The expandedfraction flows via the valves V2 and V3 to the regenerator chambers Azand A2, and thence to atmosphere via the ports P2 and P3, The fraction 0is liquefied by heat exchange with the expanded fraction in the condenser 7..

The equivalent proportion b, after leaving the main stream at D, flowsthrough valve V15 into the regenerator chamber A5, through which itflows in reverse direction to the stream through the chamber A1. thechamber A5 through the port P5, the stream b splits at X. The main partb1 flows through a valve VM and pipe 10 to join the stream a at C. Asubsidiary part b2, controlled by a constant flow regulator 11, flows byway of a valve P4 into the regenerator chamber A4 in order to build upthe pressure in this chamber for a purpose described below. I

The rotary valve I slowly rotates so as,- for example, to bring the portP1 successively into line with theinlets' 0f the re'generator chambersA1 (as shown), A5, A4, As, An, A1 etc. Thus the infiowing air from thepre-cooler 3 is passed, for a predetermined interval of time, throughthe chamber A1, then for the same interval through the chamber A5, andso on until finally it is again'passed through the chamber A1. I

The function of the stream b2 can now be explained. In the position ofthe rotary valve 1 next succeed'mg that shown in Figure 4, it will benecessary for the stream b to pass through the valve V14 intothe'chamber A4. In 'order to ensure that this valve is already opened,the

. regenerator chamber A4 is slowly filled with dry air provided by thestream b2. When the pressure in. A4

almost reaches the manifold pressure, the valve V14 drops 7 V13, V14 andV15 is provided with a cylindrical exten v sion 32 constituting a pistonwhich is slightly smaller in diameter than the tube 33 in which it isfree to move;

The annular space between the tube and the piston constitutes arestriction, and the pressure drop across the restriction tends to raisethe piston, thus closing the valve. The value of the restriction is suchthat the valve remains open against the by-passing stream b, but closeswhen the flow slightly exceeds this value, as will happen in the caseofV14 when the port P2 reaches A4 and air from the manifold 4 rushes outto the atmosphere via V14, A4 and P2. 1 1

Condensed liquid and a slight excess of gas are continually andautomatically drawn off from the condenser 7 by means of an automaticexpansion valve 12 which is adjusted to give an output pressure slightlyabove atmospheric pressure, so that the flow rate can be con- Afterleaving trolled by means of an orifice 12a of normal dimensions. Theliquid air is delivered to an automatic float trap 13 of conventionaldesign, which discharges at intervals into a suitable receptacle. Excessair escaping with the liquid is returned to the turbine exhaust pipe 6by way of a pipe 14.

It has previously been mentioned that a feature of the invention residesin the construction and design of the expansion turbine 5. In theapparatus of Figure 4, the

turbine is an axial flow impulse turbine, the rotor, stator, shaftingand one bearing of which are shown in Figure 5. A shaft 15 upon which ismounted the rotor 16 also carries a centrifugal blower 17 which acts asthe brake 18 shown in Figure 4. The blower is shown as a schematic blockin Figure 5, since its design forms no part of the present invention. Itmay have the form of a drum having radial holes for air flow. Highpressure air is fed via the inlet 19 to the stator blades 20 having ashroud ring 21 shrunk thereon to avoid tip leakage. A turbine accordingto the invention has been constructed, in which the turbine rotor ismachined from a solid piece of brass and having a tip diameter of aboutthe rotor blades being about A long and having a chord width of A". Theshroud ring is of brass and the rotor is mounted on the shaft 15 whichis of silver steel having a diameter of This shaft is about 4" long andis mounted in cup bearings at each end. The shaft 15 is machinedconically at its end which seats in balls 22 located in the cup 23; thestalk 24 of the cup being hollow to allow for the passage of oil to thebearing.

An annular shoulder of the cup 23 bears on a radial compression spring25, which in turn bears on the bottom of a cup 26. This spring 25 takesup the normal amount of differential expansion and effectively maintainsa substantially constant load on the ball bearings thereby preventingplay between the conical end of the shaft and the outer case of thehearing. A stem 27 integral with the cup 26 passes through an externallythreaded sleeve 28 as shown, which sleeve is adapted to pass through ahole in the outer case (not shown) and to have a nut screwed thereon tosupport the shaft and bearing assembly within the casing. The stem 27 ishollow, its bore communicating with the bore through the hollow stalk 24for the supply of oil thereto.

A small clearance is allowed between the cup 26 and the internal wall ofthe sleeve 28. Thus the hearing as a whole is capable of radialdisplacement, which is damped by means of two oil-filled dashpotmechanisms of which only one (29) is shown in the drawing and which aremutually at right angles. These dashpots are of conventional design, andthe pistonrods 30 are connected to the cup 26 through a close-fittingring 31. The bearing at the other end of the shaft 15 may be similarlyconstructed or, alternatively, may be similar except that no allowanceis made for radial displacement.

In an alternative construction of the turbine, the cup 26 may bereplaced by a sliding collar to which are attached the piston rods ofthree-dashpots at 120 to each other. In another construction, the pistonrods are secured to the stem 27 instead of being attached to the cup 26.

If the regenerator chambers A1 to A are to be of the small sizenecessary to keep the bulk of the machine as small as possible, theyrequire a packing material which is eflicient and simple to constructand has a high conductivity in the direction transverse to the directionof flow therethrough. A low thermal conductivity in the direction of theflow is desirable. In the present invention, it is preferred to use apacking consisting of discs of fine wire gauze, which discs areseparated by discs of thermally insulating material such as cloth ofopen weave, for example, rayon net.

What I claim is:

l. The method of liquefying a gas comprising compressing the gas,dividing the compressed gas into two parts, a major part and a minorpart, passing at least the major part through a first stage cooler tocool it, dividing the major part, after such cooling into two parts, asmall part and a large part, after such division passing the small partthrough a condenser to liquefy it and after such division combining thelarge part with the minor part which is uncooled, expanding the combinedparts adiabatically accompanied by the expenditure of energy to reducetheir temperature, and passing the expanded combined parts first throughthe condenser in counterfiow with respect to the small part liquefiedtherein and secondly through the first stage cooler in counterfiow withrespect to the compressed gas flowing through it, the mass flow per unittime of compressed gas flowing in one direction through the first stagecooler being substantially equal to the mass flow per unit time of gasespassing through the first stage cooler in the other direction.

2. The method according to claim 1 wherein the division of thecompressed gas into the major part and the minor part takes place beforepassing through the first stage cooler, the minor part being expanded asaforesaid along with the large part of the major part, Without the minorpart passing through the first stage cooler.

3. The method according to claim 1 wherein the whole of the compressedgas is passed through the first stage cooler before division into themajor part and the minor part, the minor part being then passed throughthe first stage cooler in counterflow with respect to the said whole ofthe compressed gas where the minor part is restored to the uncooledcondition before expansion along with the large part.

4. The method according to claim 1 wherein the said expanded combinedparts are combined with vapour rising from the surface of the liquefiedgas before passing through the condenser in counterflow as aforesaid.

5. The method according to claim 2 wherein the said expanded combinedparts are combined with vapour rising from the surface of the liquefiedgas before passing through the condenser in counterflow as aforesaid andwherein the mass flow per unit time of the expanded combined partstogether with the said vapour, passing through the first stage cooler incounterfiow as aforesaid, is substantially equal to the mass flow perunit time of the major part which passes through the first stage cooler.

6. The method according to claim 3 wherein the said expanded combinedparts are combined with vapour rising from the surface of the liquefiedgas before passing through the condenser in counterflow as aforesaid andwherein the mass flow per unit time of the expanded combined partstogether with the said vapour, passing through the first stage cooler incounterflow as aforesaid, is substantially equal to the mass flow perunit time of the whole of the compressed gas passed through the firststage cooler as aforesaid minus the mass flow per unit time of the minorpart.

7. The method according to claim 1 wherein the expansion of the saidcombined parts is effected in an impulse turbine from which the expandedcombined parts emerge at a temperature at least a few degrees abovetheir liquefaction point.

References Cited in the file of this patent UNITED STATES PATENTS1,027,863 Linde May 28, 1912 1,264,845 Norton Apr. 30, 1918 1,901,389Hazard-Flamand Mar. 14, 1933 2,165,994 Zerkowitz July 11, 1939 2,529,880McClure Nov. 14, 1950 FOREIGN PATENTS 1,086 Great Britain Jan. 24, 1916of 1915

1. THE METHOD OF LIQUEFYING A GAS COMPRISING COMPRESSING THE GAS,DIVIDING THE COMPRESSED GAS INTO TWO